Installation for preparing wood fibers for a cultivation substrate

Abstract

The installation comprises a defibration sheath with an inlet for wood chips, an outlet for fibers, and two parallel rotative screws. The screws mesh with each other via their respective threads, which have at least an upstream and a downstream series of segments, with a drive upstream zone having forward threads, and a braking downstream zone having reverse threads with notches that are smaller in the braking downstream zone of the downstream series than in the braking downstream zone of the upstream series. The number of notches per thread in each braking zone is 2 to 6, and the section-to-output ratio R SO of the sum of the sections of the notches of a thread of the braking downstream zone of the downstream series to the fiber output rate is 60 mm 2 /m 3 h −1 to 80 mm 2 /m 3 h −1 .

What is claimed is: 
     
       1. An installation for preparing wood fibers for use in the composition of a cultivation substrate, which installation comprises a defibration sheath having an inlet for wood chips and an outlet for the fibers obtained from said chips, the sheath containing two parallel screws suitable for being driven in rotation in such a manner as to mesh with each other via their respective threads, said threads having, in succession, in the direction going from upstream to downstream, at least an upstream series of segments and a downstream series of segments, each of which comprises a drive upstream zone having forward threads, and a braking downstream zone having reverse threads, the threads in the braking zones having notches, the notches in the threads of the braking downstream zone of the downstream series being smaller than the notches in the threads of the braking downstream zone of the upstream series, the number of notches per thread in each braking zone lying in the range 2 to 6, and the section-to-output ratio of the sum of the sections of the notches of a thread of the braking downstream zone of the downstream series to the fiber output rate lying in the range 60 mm 2 /m 3 h −1  to 80 mm 2 /m 3 h −1 . 
     
     
       2. An installation according to  claim 1 , wherein, the ratio of the sum of the sections of the notches of a thread of the braking downstream zone of the downstream series to the fiber output rate lies the range 70 mm 2 /m 3 h −1  to 75 mm 2 /m 3 h −1 . 
     
     
       3. An installation according to  claim 1 , wherein the number of notches per thread in each braking zone lies the range 3 to 5. 
     
     
       4. An installation according to  claim 1 , wherein the threads also have an intermediate series between the upstream series and the downstream series, and the number of notches per thread in each braking zone is 5. 
     
     
       5. An installation according to  claim 1 , wherein the screws are suitable for turning at a speed lying in the range 250 rpm to 400 rpm. 
     
     
       6. An installation according to  claim 1 , wherein the screws are suitable for turning at a speed lying in the range 300 rpm to 380 rpm.